1. Field of the Invention
The invention relates generally to lighting systems and specifically to low voltage "bare wire" quartz halogen technology; particularly in retail lighting display systems and portable, exhibition lighting packages.
2. Description of the Prior Art
Generally, the lower the operating voltage of a lamp is, the smaller a filament that can be used. The small quartz halogen lamp filament runs at a much higher temperature than a conventional tungsten filament lamp, causing a much, whiter light to be emitted. This, in turn, gives a greatly improved color rendition, which is an obvious advantage when color balance is important. The smaller, point-of-light source also offers much more control over the reflection and focusing of the light beams. And, both luminous flux and color temperature of the halogen lamp remain constant throughout its life because it will not "blacken."
The total heat transmitted in a low voltage lamp is less, overall, than conventional systems because of greater lumens per watt efficiency. Typical lamps have dichroic coatings on multi-faceted reflectors to ensure at least 60% of the infra-red rays (really heat) are transmitted through the back of the lamp. The low voltage halogen lamp is particularly appropriate for displays which are sensitive to heat.
The higher performing, low voltage halogen lamp represents an opportunity for widespread energy savings. A 12V, 50W low voltage halogen lamp produces the same lamp output as a 150W, 120 VAC lamp. A savings as large as 60% in energy costs is possible by simple replacement of conventional lamps with low voltage halogen lamps.
"Bare wire" conductors are easy to install, route, and dismantle. Because bare wire does not have to be fixed into a ceiling, the effective ceiling of the lamp can be lowered and thus save on lighting wasted space. A flexible lighting system useful for individual exhibits is disclosed by Boekholt in U.S. Pat. No. 4,652,068 ('068). Bare wire cables can span structures, instead of being fixed to them, and fittings can be suspended anywhere on the cable. The freedom of positioning such systems is described by Lettenmaye in West German Patent DE 3709641 and European Patent Office EP 0283800. A bare wire system gains a "portable" nature because the cables are inexpensive and practically disposable. A portable low voltage track lighting system is described by Sillett in United Kingdom Patent UK 2185863.
Low voltage halogen lamp systems offer little shock hazard since the voltages employed are so low. But in the case of a 500W system using 12V, the current can be as high as 42 amps RMS, 60 amps peak. These high currents give rise to a risk of fire or burning, since poor contacts and other resistances in the wires can produce much more heat than would be expected in conventional systems.
Bare wire lighting systems were shown at the Sept., 1989, Frankfurt `Messe` show by exhibitors: Anta, AML, J.Bruck, and Lightline. The inventor observed that none had UL approval and the systems displayed were unlikely to gain such approval in their present forms because the fixtures were not guarded and there was not adequate short circuit protection.
Commercial necessities in the lamp systems industry require that Underwriter Laboratories (UL) approval be obtained for electrical products. City electrical inspectors are unlikely to accept non-UL devices in exhibition halls, and major retailers, generally will not accept non-UL inventory for their shelves. And, product liability insurance is virtually impossible to obtain without UL approval.
The nearest identifiable prior art categories to the present invention are UL-153 for portable electric lamps, and UL-1574, for track lighting systems. Neither of these categories specifically addresses the miniature dichroic lamp or exposed low voltage wires. However, the requirements of UL-1574 and parts of UL-991 were found by UL to be the correct standard to be applied. Two principal concerns for safety in any UL approval are, (1) over-current protection in the event of a short circuit, and (2) thermal problems with the high lamp operating temperatures.
The thermal problems are overcome by appropriate housings for the lamps. The present invention discloses a structure and method for providing over-current protection.
A major concern of safety engineers with regard to bare wire, or bare busing, is the accidental shorting of the two conductors with one another. Very thin and insubstantial shorts will tend to vaporize, while heavy and thick materials will easily conduct so much current that ordinary circuit breakers will shut off the power before the object can heat up much, but not usually before considerable arcing and sparking has taken place. The sparks created by a heavy short can start fires and heavy shorts can also become welded to the wire/cable.
The greatest danger in the heating of a short is in a medium weight and resistivity object coming into contact with the bare wires such that only modest over-currents are produced. Such objects will heat up to dangerous temperatures and can cause serious burns. Jewelry in particular is of concern. Such items as rings, bracelets, and necklaces can come into contact with bare wires. Since jewelry is worn about the body, a superheated piece of metal jewelry cannot be taken off quickly enough to avoid serious injury.
A circuit interruption device that is more sensitive and that responds much faster than conventional circuit breakers or fuses is needed in bare wire systems. Delay times on the order of the length of time a fuse's metal needs to vaporize, or a circuit breaker needs to trip can be enough time to substantially heat up a shorting object; and are therefore wholly unacceptable. Low voltage quartz halogen track lamp systems, in particular, are hard to fuse because a variable number of lamps may be placed on a track and the lamps draw much more current when starting up than they do during normal operation; the temptation in those situations is to over-fuse by some multiple, usually three or four to one--which would make the shorting hazards more acute.
The prior art has recognized the need to compensate for the higher currents that normally occur during lamp ignition start-up that mimic short circuits. Automatic compensation is provided in the start-up circuits described by Wisbey in U.S. Pat. No. 4,206,385 ('385). Also included is a circuit to interrupt the power supply to the ballast in the event that no current is flowing through the lamp, once the system has been energized.
Load sensing for unloaded or improperly loaded conditions is conventional and represented in U.S. Pat. No. 4,010,342 by Austin ('342). Varying the sensitivity of the load sensing circuit according to the power operating level of the inverter is also included in Austin ('342). Such load sensing can also include average load sensing means for providing a shutdown signal in response to the average value of the output current exceeding a predetermined maximum average output value, as in Gilliland, U.S. Pat. No. 4,785,149 ('149) and peak and average current sensing in Gilliland, United States Patent 4,716,274 ('274).
A high frequency, 30 KHz, "low-voltage" 30V RMS, fluorescent lighting system is disclosed in Nilssen, U.S. Pat. No. 4,634,932 ('932). An electronic means for preventing the power supply current from exceeding a predetermined relatively modest magnitude and a pair of conductor wires adapted to permit a number of lighting units is included in Nilssen ('932).